Thermoplastic resin foams are light weight, exhibit superior heat resistance and flexibility and are readily heat-processable into shapes. Such thermoplastic resin foams have thus achieved wide use as various cushioning, packaging and heat-insulating materials.
In the sporting area, various protectors have been proposed which serve to protect a part of a human body from a high impact force. These types of protectors generally incorporate a layer of thermoplastic resin foam as a shock absorber. If a thermoplastic resin foam is to be used as a shock absorber, it must have the ability to absorb a large part of energy when compressed in its thickness direction. This accordingly requires that the thermoplastic resin foam should have a high compressive strength in its thickness direction. Also because a human body has a contoured surface, it is desired that the thermoplastic resin foam layer is flexible enough to readily conform to the contours of the human body, i.e., has a low flexural modulus.
Conventionally, methods have been proposed for manufacture of a foam which has the increased compressive strength only in its thickness direction. For example, Japanese Patent Laying-Open No. Hei 9-150431 discloses a method in which foaming of an expandable thermoplastic resin sheet is effected after it has been laminated with a second sheet having a sufficient strength to suppress planar expansion of the expandable thermoplastic resin sheet. Accordingly, the second sheet acts to suppress planar expansion of the expandable thermoplastic resin sheet while being expanded. Hence, as foaming proceeds, the expandable thermoplastic resin sheet expands nearly one-dimensionally in its thickness direction, so that foam cells are shaped into spindles extending in the thickness direction.
The above-described prior method successfully increases the compressive strength in the thickness direction but fails to increase the compressive strength in the planar direction. The resulting thermoplastic resin foam is expected to have a high compressive modulus but a low flexural modulus. However, the presence of the second sheet which acts to suppress planar expansion of the expandable thermoplastic resin sheet actually increases a flexural modulus of the laminate. Consequently, its flexibility has been insufficient.
In Japanese Patent Laying-Open No. Hei 10-44178, a method is disclosed for allowing an expandable thermoplastic resin sheet to undergo pseudo-one-dimensional expansion in its thickness direction by devising its shape. However, the method described in this prior reference results in the formation of high-density fusion bond interfaces which extend in the thickness direction of a resulting foam. These interfaces increase a flexural modulus of the foam, making its flexibility insufficient. That is, this prior art failed to reduce a flexural modulus of a resulting foam while increasing its compressive strength across the thickness.
In Japanese Patent. Laying-Open No. Hei 5-208421, a method is disclosed for manufacturing a composite foam product by integrating a non-foamable layer with a foamable layer. This prior art contemplates to obtain the composite foam product using clean materials which maintain high fluidity even in such conditions that a pressure and a shear force are little exerted thereon. For this purpose, this reference discloses a method in which the non-foamable layer consisting of thermoplastic elastomer powder, together with the foamable layer consisting of resin powder of ethylene-vinylacetate copolymer and a heat decomposable foaming agent, are subjected to powder molding so that they are foam processed.
However, the manufacturing method described in this prior reference simply suggests an easy way to manufacture the composite foam having a complex shape without a residual strain. That is, the resulting composite foam has a foam layer which is different from that produced via one-dimensional expansion of the foamable layer in its thickness direction. Accordingly, a preferential and sufficient increase in compressive strength of the composite foam in its thickness direction does not result.